The present invention relates to a positive electrode for a lithium rechargeable electrochemical cell and including a current collector made of aluminum. The invention also extends to a method of manufacturing such a cell and to any electrochemical system including such an electrode.
Lithium rechargeable electrochemical cells possess a positive electrode of the paste type comprising a conductive support which serves as a current collector on which there is deposited a paste containing a binder and an electrochemically active material capable of inserting lithium into its structure. The cathode active material is generally a transition metal compound such as an oxide, in particular a lithiated oxide, a sulfide, or a sulfate.
By way of example, the current collector of the positive electrode can be a foil, a perforated foil, a grid, an expanded metal, a felt, or indeed a foam. It is possible to use, in particular, stainless steel, titanium, aluminum, nickel, etc.
The paste is made by putting the binder in solution or suspension in a solvent and then adding it to the active material powder. After coating with the paste, the solvent is eliminated by drying the electrode. Most cathode binders commonly in use today are implemented in an organic solvent. This applies in particular to polyvinylidene fluoride (PVDF) which is dissolved in N-methylpyrrolidone (NMP). However, methods involving organic solvents present drawbacks when implemented at an industrial scale because of the toxicity of the solvents used and because of problems of expense and security associated with recycling a large volume of solvent. Attention has thus been given to binders that are compatible with the use of aqueous solvents.
U.S. Pat. No. 5,378,560 describes a positive electrode for a lithium cell whose support is an aluminum foil which is covered in an aqueous paste comprising the active material which is a lithiated cobalt oxide LiCoO2, a conductive material (graphite), and a binder which is a mixture of sodium polyacrylate and a polymer selected from the copolymers of styrene and butadiene that have a carboxyl group.
However, the Applicant has observed that when an aluminum collector is put into contact with a paste containing water, it suffers from corrosion. Its surface layer of alumina is attacked and gas is given off.
The positive electrode described in U.S. Pat. No. 5,580,683 possesses a metal current collector which may contain aluminum, specifically an aluminum alloy. That document also mentions using water as the solvent when making the paste. The paste is dried before being pressed on the collector. That method avoids putting the aluminum into contact with the aqueous medium of the paste, but it does not solve the problem of collector corrosion when a paste containing water is applied thereto.
In order to lower interface impedance, the surface of an aluminum current collector is covered in a protective primer layer.
U.S. Pat. No. 5,591,544 purposes treating the collector to eliminate the native alumina layer and then to cover its surface in a primer layer containing carbon and possibly also a transition metal oxide suitable for inserting lithium.
Document WO 97/14188 describes a primer layer comprising carbon and an alkali metal polysilicate, which primer is disposed between the current collector and the layer containing the active material.
Such a carbon layer is known to be porous; it does not constitute a reliable barrier against corrosion.
Furthermore, tests performed by the Applicant on commercial collectors coated in a layer of analogous protective carbon have shown that the carbon layer dissolves in the presence of an aqueous paste.
An object of the present invention is to propose a positive electrode for a lithium rechargeable electrochemical cell, in which the aluminum current collector is not corroded when a paste containing water is applied.
The present invention provides a positive electrode for a lithium rechargeable electrochemical cell, the electrode comprising an aluminum current collector and a paste containing both an electrochemically active material and a binder suitable for being put into solution or suspension in water, the electrode being wherein the surface of said collector is covered in a protective layer constituted by at least one component selected from an oxalate and a compound of at least one element selected from silicon, chromium, and phosphorus.
Most polymers can be used as the binder providing they are soluble in water or else form a stable emulsion (latex) when put into suspension in water.
By way of example, such polymers are polymers of acrylic acid (PAAc), of methacrylic acid, of sulfonic acid, of itaconic acid, salts thereof (e.g. with Li30, Na30, K30, . . . , cations), polyacrylamides (PAA), polyamides (PA), polyvinylalcohol (PVA), precursors of polyurethanes (PU), polytetrafluoroethylene (PTFE), together with copolymers thereof and mixtures thereof. It is also possible to use phenolic resins, cellulose compounds, and elastomers.
In a first embodiment of the invention, the binder comprises at least one elastomer. Suitable elastomers that can be selected include: a copolymer of acrylonitrile and butadiene (NBR); a copolymer of acrylonitrile and hydrogenated butadiene (HNBR); a copolymer of styrene and butadiene (SBR); a copolymer of styrene and acrylonitrile (SAN); a terpolymer of acrylonitrile, butadiene, and styrene (ABS); a terpolymer of styrene, acrylonitrile, and styrene (SAS); a terpolymer of styrene, isoprene, and styrene (SIS); a terpolymer of styrene, butadiene, and styrene (SBS); a terpolymer of ethylene, propylene, and diene (EPDM); a styrene/ethylene/butene/styrene (SEBS) polymer; a polyurethane thermo-plastic rubber (PUR); a polychloroprene (CR) or 2 chloro 1-3 butadiene (e.g. known under the trade name xe2x80x9cNeoprenexe2x80x9d); a polyisobutylene (PIB); a polyisoprene; a polybutadiene; an ethylene/propylene copolymer (EPR); a copolymer of ethylene and vinyl acetate (EVA); a copolymer of ethylene and acrylic ester (EMA, EEA, . . .); and mixtures thereof.
Said elastomer is preferably selected from styrene/butadiene copolymer (SBR) and acrylonitrile/butadiene copolymer (NBR).
In a second embodiment of the invention, the binder comprises at least one cellulose compound. The binder can be a cellulose compound selected from: a methyl cellulose (MC); a carboxymethylcellulose (CMC); a hydroxyethylcellulose (HEC); and a hydroxypropylmethylcellulose (HPMC).
Said cellulose compound is preferably a salified carboxymethylcellulose (CMC) having a mean molecular weight of not less than 200,000.
In a third embodiment of the invention, said binder comprises at least one elastomer and at least one cellulose compound.
In a first variant, said elastomer is styrene/butadiene copolymer (SBR) and the cellulose compound is salified carboxymethylcellulose (CMC) having a mean molecular weight of not less than 200,000.
In a second variant, said elastomer is acrylonitrile/butadiene copolymer (NBR), and the cellulose compound is salified carboxymethylcellulose (CMC) having mean molecular weight of not less than 200,000.
Said binder contains 30% to 70% by weight of said cellulose compound relative to the sum of the weight of said elastomer plus the weight of said cellulose compound.
Said binder preferably contains 50% to 70% by weight of said elastomer and 30% to 50% of said cellulose compound relative to the sum of the weight of said elastomer plus the weight of said cellulose compound.
More preferably still, said binder contains 60% to 70% by weight of said elastomer and 30% to 40% by weight of said cellulose compound relative to the sum of the weight of said elastomer plus the weight of said cellulose compound.
In addition to the binder, the paste also contains an electrochemically active material. Said active material is selected from: a transition metal oxide; a sulfide such as Li3MoS3, LiTiS2, Li3NiPS3, or Li4V2S5, for example; a sulfate, e.g. Fe(SO4)2 or Ni(SO4)2, for example; and mixtures thereof.
Said transition metal oxide is preferably selected from: vanadium oxide (V2O5); and lithiated oxides based on manganese (LiMnO2, LiMn2O4), on iron (LiFeO2), on nickel (LiNiO2), and on cobalt (LiCoO2), optionally doped by at least one element selected in particular from magnesium, calcium, strontium, barium, titanium, zirconium, vanadium, chromium, manganese, iron, cobalt, nickel, zinc, boron, aluminum, and mixtures thereof.
The invention also provides a lithium rechargeable electrochemical cell comprising an electrode, at least one negative electrode, and at least one positive electrode of the present invention.
The active material of the negative electrode is preferably a carbon-containing material suitable for inserting lithium into its structure. The active material of the anode can be selected from carbons, in particular crystallized carbons such as graphites in powder or fiber form, carbons of low crystal content that are graphitizable such as cokes, or not graphitizable such as vitreous carbons or carbon blacks, and mixtures of such carbons.
However, the negative active material can also be selected from compounds containing a metal suitable for forming an alloy with lithium, such as metallic carbides, nitrides, sulfides, and oxides.
The electrolyte can be a liquid or a polymer.
In a variant, the electrolyte is liquid and comprises at least one lithium salt dissolved in at least one organic solvent.
In another variant, the electrolyte comprises at least one polymer matrix and a lithium salt.
The lithium salt is selected from: lithium perchlorate LiClO4; lithium hexafluoroarsenate LiAsF6; lithium hexafluorophosphate LiPF6; lithium tetrafluoroborate LiFB4; lithium trifluoromethanesulfonate LiCF3SO3; lithium trifluoromethanesulfonimide LiN(CF3SO2)2 or xe2x80x9cLiTFSIxe2x80x9d; and lithium trifluoromethanesulfonemethide LiC(CF3SO2)3 or xe2x80x9cLiTFSMxe2x80x9d.
The organic solvent comprises an ether, an ester, or a mixture of ethers and/or esters, the ester being selected from linear carbonates, and cyclic carbonates having more than four carbon atoms. Said organic solvent preferably comprises at least: an ester selected from a carbonate such as propylene carbonate, ethylene carbonate, butylene carbonate, diethyl carbonate, and dimethyl carbonate; an acetate such as ethyl acetate; a propionate; or a butyrate such as methyl butyrate.
The present invention also provides a method of manufacturing a positive electrode for a lithium rechargeable electrochemical cell, the electrode having an aluminum current collector, a paste containing an electrochemically active material, and a binder.
In a first implementation of the invention, said method comprises the following steps:
a paste is made by adding an electrochemically active material in powder form to a binder in solution or in suspension in water;
to form an electrode, said paste is spread on the aluminum current collector which is covered in a protective layer constituted by at least one component selected from an oxalate and a compound of at least one element selected from silicon, chromium, and phosphorus;
said electrode is dried at a temperature lying in the range 40xc2x0 C. to 120xc2x0 C. to evaporate the water; and
said electrode is compressed at ambient temperature until porosity is obtained that preferably lies in the range 30% of the 40% of the total volume of the electrode.
In a second variant of the invention, the method comprises the following steps:
a paste is made by adding an electrochemically active material in powder form and at least one component selected from an oxalate, a silicate, a chromate, a phosphate, and a phosphochromate to a binder in solution or in suspension in water;
said paste is spread on an aluminum current collector to form an electrode;
said electrode is dried at a temperature lying in the range 40xc2x0 C. to 120xc2x0 C. to evaporate the water; and
said electrode is compressed at ambient temperature to obtain porosity that preferably lies in the range 30% to 40% of the total volume of the electrode.
Said component is preferably selected from: sodium monosilicate SiO2NaOH; sodium metasilicate Na2SiO3; potassium chromate K2CrO4; and potassium bichromate K2Cr2O7. More preferably, said component is sodium metasilicate Na2SiO3.
In a first implementation of the invention, the binder has at least one elastomer. Said elastomer is preferably selected from a styrene/butadiene copolymer (SBR) and an acrylonitrile/butadiene copolymer (NBR).
In a second implementation of the invention, the binder comprises at least one cellulose compound. Said cellulose compound is preferably salified carboxymethylcellulose (CMC) having mean molecular weight of not less than 200,000.
In a third implementation of the invention, said binder comprises at least one elastomer and at least one cellulose compound. Said cellulose compound is preferably salified carboxymethylcellulose (CMC) of mean molecular weight not less than 200,000, and said elastomer is selected from a styrene/butadiene copolymer (SBR) and an acrylonitrile/butadiene copolymer (NBR).
Said binder contains 30% to 70% by weight of said elastomer and 30% to 70% of said cellulose compound relative to the sum of the weight of said elastomer plus the weight of said cellulose compound. Preferably those figures are respectively 50% to 70% of said elastomer and 30% to 50% of said cellulose compound, and more preferably 60% to 70% of said elastomer and 30% to 40% of said cellulose compound.
Said active material is selected from a transition metal oxide, a sulfide, a sulfate, and mixtures thereof. Said active material is preferably a vanadium oxide, a lithiated oxide, or a lithiated oxide doped with a transition metal.
In addition, said paste may contain additives such as a conductive material or a texturing agent. This applies in particular to the positive electrode when the active material is a lithiated oxide of a transition metal. Since such oxides are naturally semiconductive, it is necessary to add a conductive material thereto, e.g. graphite, carbon, or soot, in order to increase the macroscopic conductivity of the electrode.